Make FindExiv2.cmake work with exiv 0.27.

Summary:
In exiv2 >= 0.27, the version is not defined in version.hpp but in exv_conf.h.
The module now:
- tries to use the version in the pkgconfig file,
- If it's not (or cannot be) found, it parses both files to find it.

The module was also refreshed to use the modern CMake syntax and follow our
CMake policies.

Test Plan: exiv2 0.26 and 0.27 are found.

Reviewers: kde-buildsystem

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D17040